1. Field of the Invention
The present invention is directed to an x-ray detector element and operating circuitry therefor for use in registering x-rays, particularly for use in computer tomography.
2. Description of the Prior Art
Various demands are made on x-ray detector elements which are used in computer tomography. Such elements should convert x-rays into electrical signals with a high sensitivity and with substantial immunity to interference. Such detectors also should have a large dynamic sensitivity range with a good proportional relationship between the incoming x-ray intensity and the level of the electrical output signal of the detector. The chronological response behavior of the detector should be rapid, and the measured signal should have substantially decayed after approximately one millisecond. Moreover, it is required that such detector elements exhibit good long-term stability.
Known x-ray detectors used in computer tomography employ various physical principles for the detection of the x-rays. Gas-filled ionization chambers make use of the ionizing property of x-rays, and the electrical charges generated in the ionized gas by the incoming x-rays can be directly identified.
Detectors operating according to the scintillator principle employ the luminous properties of various crystalline phosphors which convert the x-rays into visible light, so that the x-rays becomes visible to the human eye, or to a light-sensitive film or a photodetector.
A direct conversion of x-rays into an electrical signal is accomplished, for example, using a pn-diode of monocrystalline germanium semiconductor material. Utilization of the photovoltaic effect for direct detection of x-rays has also been proposed.
Spatially resolving x-ray detectors operating using the ionization chamber principle require a complicated manufacturing technique, and have low efficiency in the conversion of the x-rays into electrical pulses. The use of such detectors therefore requires a relatively high radiation dose, which imposed a high radiation load on the patient to be examined.
Phosphor detectors are basically well-suited for x-ray registration, but can only be structured in a relatively complicated manner. Further problems arise due to the interference-prone connection between the scintillator and the photodetector associated therewith, and electrical loses must also be accepted in the opto-electrical conversion.
Semiconductor diodes formed of germanium require cooling with liquid nitrogen because the relatively small band gap of germanium results in the generation of high thermal noise. Providing for such cooling is especially complicated, particularly in a computer tomography apparatus wherein an array of individual x-ray detectors must be moved around the body to be irradiated.